The retinal pigment epithelium (RPE) is a polarized monolayer of cells that provides essential functions to the light-sensitive photoreceptors in the retina. Many of the cell biological processes involving the RPE, including those underlying disease mechanisms, can be studied using in vitro culture systems. For such models to be informative, the RPE cultures must be well-differentiated and fully mature, exhibiting the key characteristics of their native counterparts. In this mini review, we emphasize this requirement to use fully differentiated RPE cultures by discussing structural, functional, and metabolic aspects of the RPE.